Using logical operators in an expression

I am trying to understand that bit of code:

s = list()
r = string()
# actual bit of code:
print(s and 'ERROR' or r or 'EMPTY')

What this does:

• if s is not an empty list, then you must print 'ERROR'
• else if r is not an empty string you must print r
• else print 'EMPTY'

I am now trying to understand why this works, and more widely how you can use logical operators in expressions, like in this example.

Answers:

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Method 1

x and y is x if it is “false-y” (i.e. 0, False, None, [], '', …), otherwise it is y.
x or y is x if it is “truth-y”, otherwise it is y.

That is, x or y is the same as x if x else y, and x and y the same as x if not x else y.

Method 2

If s is non-empty, s and 'ERROR' produces the truthy value 'ERROR', which short-circuited the rest of the or expression.

If s is empty, s and 'ERROR' produces the false value [], which leads to the evaluation of r to determine if the value of the entire expression is r or EMPTY.

It helps to play with smaller examples using non-Boolean arguments to see how both and and or work.

>>> [] and "ERROR"
[]
>>> ["foo"] and "ERROR"
"ERROR"

It might be clearer to use the conditional expression here:

print("ERROR" if s else (r or "EMPTY"))

or just use a “naive” if statement.

if s:
    print("ERROR")
elif r:
    print(r)
else:
    print("EMPTY")

Method 3

This is a question more focused on logical operators and Short-circuit evaluation
.
Firstly, you need to understand that every variable has a boolean value associated with it.

An empty list is false, a list with a length of 1 or higher is true. This can be checked with the bool() operator:

>>> bool([])
False
>>> bool(['a'])
True

Similarly, an empty string is false and a string of one or more characters is true.

>>> bool('')
False
>>> bool('string')
True

The expression s and 'ERROR' or r or 'EMPTY' is evaluated as follows:

1. bool(s) and bool('ERROR')
2. or
3. bool(r)
4. or
5. bool('EMPTY')

When any of these is True, the evaluation ends. It’s merely the order of evaluation.

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